J. B. A. Mitchell

Merged electron-ion beam experiments. I. Method and measurements of (e-H2+) and (e-H3+) dissociative-recombination cross sections

A merged-beam experiment designed to study collisions between electrons and molecular ions at very low energies and with high energy resolution is described. The theory behind the technique is reviewed and a comparison is made with other intersecting-beam techniques. Cross sections for dissociative recombination of H2+ and H3+ with electrons have been measured through the energy range 0.01-4 eV with an estimated energy resolution of approximately 0.04 eV. The structure found in both curves reflects the formation of Rydberg states of the molecules which in some cases choose to decay through the autoionisation and pair-production channels.

Merged electron-ion beam experiments. V. Dissociative recombination of OH+, H2O+, H3O+ and D3O+

For pt.IV see ibid., vol.14, no.8, p.1353-671 (1981). Absolute cross sections for dissociative recombination of OH+, H2O+, H3O+ and D3O+ have been measured using a merged electron-ion beam apparatus through an energy interval from 0.01 to 1 eV. The recombination cross section for the diatomic OH+ is small, one of the smallest measured by the authors for a diatomic ion. The cross sections for the polyatomics H2O+, H3O+ and D3O+ are large and nearly identical. Like most other polyatomic ions they exhibit nearly an Ecm-1 energy dependence below approximately 0.1 eV. At higher energies the slope of the cross section versus electron energy is much steeper with an energy dependence near Ecm-2. Cross sections for D3O+ are less than those for H3O+ indicating the presence of a small isotope effect.

Measurements of C3H+3, C5H+3, C6H+6, C7H+5, and C10H+8 dissociative recombination rate coefficients

The rate coefficients α for the dissociative recombination (DR) of the aromatic cyclic ions C6H6+ and C10H8+ have been measured in a flowing afterglow experiment at room temperature using Xe+ and Kr+ as precursor ions. C+ was also used as a precursor ion and this allowed the DR rate coefficients for C3H3+, C5H3+, and C7H5+ to be determined. The values obtained (in units of 10−6 cm3 s−1) at 300 K are, respectively, α(C3H3+)=0.7, α(C5H3+)=0.9, α(C6H6+)=1.0, α(C7H5+)=0.7, α(C10H8+)=0.3 with an uncertainty estimated to be ∼30%. A discussion is given concerning the relationship between the DR rate and the structural complexity of the ions.

The dissociative recombination of hydrocarbon ions. I. Light alkanes

Rate coefficients for the dissociative recombination of a series of hydrocarbon ions derived from normal alkanes have been measured using the FALP-MS technique. Despite the increasing complexity of the ions along the series, the measured rate coefficients are found to be remarkably similar having values between 5.5 and 8.3×10−7 cm3 s−1.

A further study of HCO+ dissociative recombination

The rate coefficient for the dissociative recombination of HCO+ has been measured using a new flowing afterglow technique which employs a movable Langmuir probe to measure electron density and a movable mass spectrometer to measure ion density, both as a function of distance along the flow. A value of 2.2×10−7 cm3 s−1 has been found at 300 K. An analysis of the excitation state of the ions indicates that more than 93% are in the v=0 state while the rest have ∼0.1 eV of internal energy. A discussion of recent theoretical controversy concerning this ion is given.

Further measurements of the H+3(v=0,1,2) dissociative recombination rate coefficient

A new flowing afterglow apparatus that utilizes a Langmuir probe/mass spectrometer to monitor both electron and ion decay in a hydrogen plasma has been used to measure the dissociative recombination rate coefficient of H+3 at two different electron temperatures. At 300 K a rate coefficient of 1.5×10−7 cm3 s−1 was found for H+3 ions with a low degree of vibrational excitation (v≤2). The rate coefficient for ground state ions H+3(v=0) was measured as 1.1×10−7 cm3 s−1 at 650 K. A discussion is given of the excitation states of H+3 ions in the afterglow in the light of slow deexcitation rates for low vibrational states. A new model for the recombination of H+3 is presented.

Experimental studies of cold electron attachment to SF6, CF3Br, and CCl2F2

A new technique has been used for the measurement of electron attachment rate coefficients for the molecules, SF6, CF3Br, and CCl2F2 at temperatures between 48 and 170 K. The results demonstrate very clearly the strong effect that internal vibrational energy of the molecules has on the attachment process.

Merged electron-ion beam experiments. IV. Dissociative recombination for the methane group CH+,...,CH5+

The dissociative recombination of the methane-derived group, CH+,...,CH5+, has been measured over the centre-of-mass energy interval 0.03 to 2 eV. Assuming a Maxwellian distribution of the electron velocities, the derived rate coefficients for CH+,...,CH5+ at 100K are, respectively, 5, 8.7, 12, 12.8 and 12.8*10-7 cm3 s-1. The recombination cross section for the diatomic ion CH+ is large. This has far reaching consequences for the theories of molecular formation in interstellar clouds. The cross sections for the polyatomics CH2+,...,CH5+ are also large and roughly equal in magnitude. Like CH+ they exhibit a close to E-1 energy dependence below 0.1 eV. For CH3+, CH4+ and CH5+ the slope of the cross sections increases above 0.1 eV.

New FALP-MS measurements of , and dissociative recombination

A series of measurements of the rate coefficient for the dissociative recombination (DR) of has been recorded. A value of was found for ions that are vibrationally cold. This is somewhat smaller than that measured previously by this group. The difference is ascribed to aerodynamic effects which affected our earlier measurement that have been eliminated in the present work. Results for and DR are also reported.

The dissociative recombination of hydrocarbon ions. II. Alkene and alkyne derived species

Dissociative recombination reactions of a number of cyclic ions, derived from ion-molecule reactions involving acetylene as parent molecule and of protonated butane derived from reactions involving ethylene have been studied. It is found that the recombination rate coefficient does not seem to be much affected by the substitution of functional groups for hydrogen atoms in the cyclic cases. Also, protonated butane has a rate coefficient very similar to that for protonated methane.